This invention relates to apparatus for batch processing wafers of semiconductor or similar type material, and more specifically to such apparatus which permits batch processing of the wafers with improved uniformity, throughput and yield.
Polishing an article to produce a surface which is highly reflective and damage free has application in many fields. A particularly good finish is required when processing an article such as a wafer of semiconductor material in preparation for printing circuits on the wafer by an electron beam-lithographic or photolithographic process. Flatness of the wafer surface on which circuits are to be printed is critical in order to maintain resolution of the lines, which can be as thin as 1 micron or less.
Flatness is quantified using a number of measuring methods. For example, "Taper" is a measurement of the lack of parallelism between the unpolished back surface and a selected focal plane of the wafer. "TIR", or Total Indicated Reading, is the difference between the highest point above the selected focal plane and the lowest point below the focal plane, and is always a positive number. "FPD", or Focal Plane Deviation, is the highest point above, or the lowest point below, the chosen focal plane and may be a positive or negative number. "TTV", or Total Thickness Variation, is the difference between the highest and lowest elevation of the polished front surface of the wafer. Presently, flatness of the polished surfaces of wafers is not significantly improved, and may be worsened, by the polishing process. In batch processing using currently known polishing apparatus, there will be a significant number of wafers which fail to meet flatness and polishing specifications after polishing, thus adversely affecting yield in commercial production.
Conventional batch polishing machines include a disc-shaped polishing pad mounted on a turntable for driven rotation about a vertical axis passing through the center of the pad. The wafers are fixedly mounted on a pressure plate above the polishing pad and forced into polishing engagement with the rotating polishing pad. A polishing slurry, typically including chemical polishing agents and abrasive particles, is applied to the pad. In order to achieve the degree of polishing needed, a polisher head is driven against the pressure plate by a hydraulic cylinder to apply a substantial normal force to the plate for pressing the wafers into polishing engagement with the pad.
Batch wafer processing using the conventional batch polishing machine described above has a number of associated disadvantages. For example, the wafers tend to become tapered toward their peripheral edges during batch polishing. Also, where the surface of the polisher head and/or the pressure plate contains non-uniformities, such as where the surface contains dirt build-up or contains irregularities (e.g., bumps, cavities, etc.) caused during manufacture or subsequent damage, the driving force of the polisher head is non-uniformly transferred to the pressure plate. This results in non-uniform pressure between the wafers and the polishing pad, causing a deterioration of the flatness and parallelism of the wafers and a lack of consistency between wafers of the same batch.
Moreover, during the polishing process, the coefficient of friction between the polishing pad and the wafers is quite high, oftentimes in the vicinity of two. This friction causes substantial temperature increases, including in the polisher head, which can result in deformation of the polisher head surface. This leads to the same non-uniform force transfer problems discussed above. To minimize this risk, water or other coolant must be circulated in the polisher head to continually cool the polisher head surface and reduce the deformation of the surface. This circulation system thus increases the complexity and cost of manufacturing the conventional polishing apparatus.
It is known to provide fiberglass or felt rings between the polisher head and the pressure plate to overcome the tapering of the peripheral edges of the wafers during batch processing. The ring has a diameter smaller than the pressure plate and polisher head diameter so that the driving force of the polisher head is transferred to the pressure plate in an annular pattern inward of the edges of the pressure plate. This causes the pressure plate to bend upward slightly at its edges, thereby reducing the tapering of the wafers at the edges of the wafers. However, because they are solid, the material from which the ring is constructed cannot be redistributed to other portions of the ring when it is deformed (e.g., compacted) by non-uniformities in the surfaces of the polisher head and pressure plate. Thus, a localized force is transferred through the ring to the pressure plate, causing non-uniform pressure driving the wafers against the polishing pad. In addition, these rings cannot be produced with a sufficiently uniform compressive strength and thickness to transmit a uniform pressure over the entire area of contact between the polisher head and the pressure plate.
The problems of yield associated with batch processing are somewhat alleviated by single wafer processing, in which each wafer has its own polisher head. In my previously issued U.S. Pat. No. 5,193,316, I disclosed a fluid filled bag that can be used for processing a single wafer and reduces the effect of surface non-uniformities. The bag is generally equal in size to the wafer and is placed between the wafer and an individual piston head that forces the wafer against the polishing surface so that non-uniformities of the piston surface do not negatively affect polishing of the wafer. Single wafer processing also eliminates the problems of forces transmitted through the pressure plate from one wafer to another. However, this fluid filled bag cannot be used for batch polishing, such as by being placed between the polisher head and the pressure plate, because a uniform transfer of force from the polisher head over substantially the entire surface of the pressure plate would result in tapering of the peripheral edges of the wafers in the same manner as described above with respect to the conventional polishing apparatus. In addition, single wafer polishing has a very low throughput because only a single wafer per pressure plate is polished at a time.